Apparatuses, systems and methods for obtaining information about electromagnetic energy emitted from the earth, such as for locating an interference source on earth

ABSTRACT

An observation satellite is used for obtaining information about electromagnetic energy emitted from the earth. The observation satellite orbits the earth in an orbit having an inclination larger than 90° and smaller than 270°. Further, the observation satellite comprises at least one receiving antenna, the at least one receiving antenna having a receiving pattern directed towards the earth, and suitable for receiving electromagnetic energy in the radio frequency range as the observation satellite is orbiting relative to the surface of the earth. The observation satellite also comprises a transmitter configured for at least one of: (i) retransmitting, to a relay spacecraft, the received electromagnetic energy, (ii) transmitting, to the relay spacecraft, information representing the received electromagnetic energy, and (iii) transmitting, to the relay spacecraft, information derived from the received electromagnetic energy. The invention also relates to systems and methods therefor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/648,931, filed Jun. 2, 2015, which is a a national stage ofPCT/EP2013/074372, filed Nov. 21, 2013, which is a continuation-in-partof U.S. patent application Ser. No. 13/705,566, filed Dec. 5, 2012,which are hereby incorporated by reference in their entirety.

FIELD OF TECHNOLOGY

The present invention notably relates to telecommunications and toobtaining information about electromagnetic energy emitted from asource, or from sources, on the earth. The invention may for example beused for locating an interference source on earth, although theinvention is not limited to this particular application.

BACKGROUND

Satellite communications notably involve the transmission of signalsfrom a station on earth towards a satellite and vice versa. Satellitecommunications may be intended for providing communication servicesbetween two points on earth. This includes point-to-point services (e.g.internet, satellite phones) and point-to-multipoint (broadcast) services(e.g. TV). The stations engaged in satellite communications may be fixed(e.g. rooftop dish) or mobile (e.g. vehicles, ships, planes, hand-helddevices). With the increase use of satellite and terrestrialcommunications, the risk of interferences between differentcommunications also increases.

In the past, there have been numerous efforts to locate interferingtransmission stations and, more generally, there have been numerousefforts to resolve interference issues and to alleviate the disruptionscaused thereby (including reduction of the usable capacity of asatellite communication system).

A known approach to locate an interfering transmission station on earthis to use ground stations. For example, U.S. Pat. No. 5,008,679 relatesto a method of locating an unknown radio-frequency transmitter usingcorrelations between signals received by different satellites. However,performing ground-based geolocation is difficult because it relies onsignal correlation between multiple satellites and multiple groundstations: a time intensive process with many opportunities for errorpropagation within the calculation. Furthermore, the ability to performground-based geolocation is dependent on many factors, including theexistence of and extensive knowledge of adjacent satellites, theexistence of and extensive knowledge of known reference signals, and theexistence of necessary ground hardware, and as a result, ground-basedgeolocation is ineffective in many cases.

Another known approach uses helicopters or unmanned aerial vehicles(UAVs) for geolocation. However, this approach is resource inefficientand only offers one-off analyses.

In view of the above, there is a need to reduce the amount ofinterference in the context of satellite communications or, when this isnot possible, to adopt techniques to cope with such interferences.

SUMMARY

To meet or at least partially meet the above-mentioned need,apparatuses, systems and methods according to the invention are definedin the independent claims. Particular embodiments are defined in thedependent claims, and are explained in the present description.

In one embodiment, a satellite, hereinafter referred to as “observationsatellite”, is used for obtaining information about electromagneticenergy emitted from a source, or from sources, on the earth. Theobservation satellite orbits the earth in an orbit having an inclinationlarger than 90° (i.e., 90 degrees) and smaller than 270° (i.e., 270degrees), such an orbit being defined as retrograde to the direction ofthe Earth's natural rotation. Further, the observation satellitecomprises at least one receiving antenna and a transmitter. The at leastone receiving antenna has a receiving pattern directed towards theearth, and is suitable for receiving electromagnetic energy in the radiofrequency range as the observation satellite is orbiting relative to thesurface of the earth. The transmitter is configured for at least one of:(i) retransmitting at least part of the received electromagnetic energy,(ii) transmitting information representing at least part of the receivedelectromagnetic energy, and (iii) transmitting information derived fromat least part of the received electromagnetic energy.

Thus, the observation satellite is able to gather data whichsignificantly improves the capability to understand the composition andorigin of energy transmitted into space for the purposes of satellitecommunications. Given that the majority of communications satellites aredesigned for communications rather than data-gathering or analysisfunctions, an observation satellite can provide radically more data,which cannot be gathered by communication satellites as they existtoday, about the nature of uplink transmissions. Interference detection,system optimization, spectrum planning and many other applicationsbenefit greatly from the data and understanding produced by theobservation satellite. In particular, by placing the observationsatellite in a highly inclined (retrograde) orbit, its velocity relativeto the surface of the earth is higher than that of a satellite in aprograde orbit at a similar altitude and the observation satellite isable to frequently revisit a given position relative to the Earth'ssurface and receive transmissions from a certain area on the Earth.Furthermore, placing, in one embodiment, the observation satellite at analtitude similar to the altitude of satellites which transmissions ofinterest are generally intended for (hereinafter referred to as “targetsatellites”) is advantageous in that the observation satellite is thenable to observe a large area on Earth and observe most signals intendedfor certain target satellites.

The use of the above-mentioned observation satellite for understandingand handling interference issues in the context of satellitecommunications results from a change of focus. The aim is to attempt tofind out which signals are emitted into space from sources on earth.However, rather than attempting to do so from a point near the earthsurface, the observation satellite enables to do so from a point intospace and especially from a point near the orbit of interest. This thusenables information to be gathered about the signals reaching the orbitof interest more effectively.

The inclination is the angle between the equatorial plane and the planein which the satellite orbits the earth. A satellite with an inclinationof 0° (i.e., 0 degree) is defined as orbiting in the equatorial plane indirection of the rotation direction of the earth.

As mentioned above, the observation satellite's transmitter isconfigured for performing one of three operations (denoted (i), (ii) and(iii) respectively), a combination of two of these three operations(namely, (i)+(ii), (i)+(iii), or (ii)+(iii)), or all three operations(namely, (i)+(ii)+(iii)).

Namely, in a sub-embodiment, the transmitter is configured to retransmitat least part of the received electromagnetic energy (for example to aground station on earth, or to another satellite). Hence, the receivedelectromagnetic energy, or a part thereof, may be transmitted withoutany processing by the observation satellite itself. In that case, anyprocessing of the received electromagnetic energy, or a part thereof,(for analysis, geolocation, etc.) may then be performed outside theobservation satellite, for example on earth, by a processing station.

Alternatively, or additionally, in another sub-embodiment, thetransmitter may be configured to transmit (for example to a groundstation on earth, or to another satellite) information representing atleast part of the received electromagnetic energy. This may, forexample, mean digitizing and/or compressing the received electromagneticenergy, or a part thereof.

Alternatively, or additionally, in yet another sub-embodiment, thetransmitter may be configured to transmit (for example to a groundstation on earth, or to another satellite) information derived from atleast part of the received electromagnetic energy (especially instead ofthe received electromagnetic energy itself). The information may bederived from the received electromagnetic energy, or from a partthereof, for example by determining or estimating, using some equipment(also called “payload”) within the satellite itself, the location onearth of the source of received electromagnetic energy (such as thesource of an interfering source). Deriving information from the receivedelectromagnetic energy, or from a part thereof, may also comprisegenerating, using some equipment within the satellite itself,information on the energy spectrum, polarization, modulation scheme,etc. of the received electromagnetic energy.

In other words, the observation satellite is used for obtaininginformation about electromagnetic energy emitted from a source, or fromsources, on the earth in the sense that the process of actuallyobtaining the information about the received electromagnetic energy maybe performed within or outside the observation satellite. Namely, theobservation satellite may act as a tool contributing to the process ofobtaining information about electromagnetic energy emitted from asource, or from sources, on the earth.

In one embodiment, the observation satellite is suitable for obtaininginformation about electromagnetic energy emitted from a source, or fromsources, on the earth and reaching the geostationary orbit. Monitoringsignals transmitted towards satellites in the geostationary orbital arcat an altitude of roughly 35800 kilometers is particularly interestingsince the geostationary orbit is of paramount importance for numeroussatellite communications.

In one embodiment, the observation satellite's transmitter is configuredto transmit, towards the earth, at least part of the receivedelectromagnetic energy or the information representing, or derived from,at least part of the received electromagnetic energy. Doing so enables areceiving station on earth, or a processing station communicating withsuch receiving station, to derive further information from theelectromagnetic energy received by the satellite. For example, thelocation of the source of some received electromagnetic energy may bedetermined.

The transmitter may however not be configured to directly send thereceived electromagnetic energy (or a part thereof) or the informationrepresenting, or derived from, the received electromagnetic energytowards the earth. Instead, another satellite or spacecraft may functionas a relay station relaying the transmission to earth. Such relaysatellite or spacecraft may be configured to perform further processingof information received from the observation satellite. The observationsatellite may transmit, to the relay satellite or spacecraft, using oneor more optical communication link.

In one embodiment, the information derived from the receivedelectromagnetic energy is obtained by processing at least part of thereceived electromagnetic energy within the observation satellite. Insuch case, processing equipment is integrated with the observationsatellite to do so.

In one embodiment, the processing comprises at least one of: (a)selectable down-conversion of analog signal to common intermediatefrequency; (b) analog-to-digital conversion of signals provided by, i.e.carried by, at least part of the received electromagnetic energy; (c)spectrum analysis of at least part of the received electromagneticenergy; (d) Doppler shift analysis of at least part of the receivedelectromagnetic energy; (e) Doppler rate analysis of at least part ofthe received electromagnetic energy; (f) direction of arrival or angleof arrival processing; (g) time difference of arrival (TDOA) processing;(h) frequency difference of arrival (FDOA) processing; (i) referencemeasurements between two or more antenna elements; (j) data filtering;and (k) data compression.

Thus, the observation satellite may be capable of performing differentkinds of processing on the received electromagnetic energy.

For example, through the analog-to-digital conversion, the signalscarried by the received electromagnetic energy become accessible forfurther digital processing.

Spectrum analysis and direction of arrival processing (or directionfinding (DF)) may also help to identify the composition and the originof the electromagnetic energy respectively. A skilled person would knowhow to perform spectrum analysis and direction-of-arrival (DOA)processing. In that respect, more background about DOA processing mayfor example be found in Schmidt, R. O., “Multiple emitter location andsignal parameter estimation,” IEEE Transactions on Antennas andPropagation, Vol. 34, No. 3, 276-280, March, 1986; or in Lipsky, StephenE., “Microwave passive direction finding”, SciTech Publishing, 2003.Numerous other references exist in the academic literature regardingdirection-of-arrival processing.

Data filtering and data compression could reduce the data amounttransmitted by the observation satellite.

In one embodiment, the at least one receiving antenna of the observationsatellite is suitable to receive electromagnetic energy in a radiofrequency range between 1 GHz and 100 GHz. Obtaining, and thenanalyzing, electromagnetic energy in the microwave range (i.e., between1 GHz and 100 GHz) is particularly advantageous since satellitecommunications are generally performed in this range so thatinterference in the context of satellite communications is mainly causedby electromagnetic energy in this frequency range.

In one sub-embodiment of the previously mentioned embodiment, the atleast one receiving antenna of the observation satellite is suitable toreceive electromagnetic energy in a radio frequency range being at leastone of: (a) between 1 and 2 GHz (L-band); (b) between 2 and 4 GHz(S-band); (c) between 4 and 8 GHz (C-band); (d) between 8 and 12 GHz(X-band); (e) between 12 and 18 GHz (K_(u)-band); and (f) between 26.5and 40 GHz (K_(a)-band). These exemplary frequency bands are ofparticular interest for satellite communications.

In one embodiment, the at least one receiving antenna of the observationsatellite is suitable to receive electromagnetic energy in a radiofrequency range used by geostationary satellites to receive signals fromthe earth. In other words, the observation satellite may be used forobtaining information about electromagnetic energy emitted from theearth for the purpose of satellite communications whatever the radiofrequency.

In one embodiment, the at least one receiving antenna of the observationsatellite is suitable to receive electromagnetic energy having at leastone of: a linear polarization; a vertical polarization; a horizontalpolarization; an elliptical polarization; and a circular polarization.This embodiment is advantageous since it may be useful to obtaininformation about the polarization of the received electromagneticenergy.

In one embodiment, the at least one receiving antenna of the observationsatellite is configured to receive, during one orbital period,electromagnetic energy from an area covering more than half of thesurface of the earth.

In one embodiment, the received electromagnetic energy comprises morethan only information intended for controlling the observation satelliteitself. The observation satellite indeed aims at gathering informationabout signals intended to satellites other than the observationsatellite. The observation satellite aims at gathering information aboutsignals intended to the so-called target satellites, as mentioned above.

In one embodiment, the received electromagnetic energy comprises energyintended for at least one other satellite. The signals for controllingthe observation satellite itself may be transmitted in a specificfrequency band, or through other means as will be explained thereafter.Thus, in this embodiment, the electromagnetic energy received by theobservation satellite is not a control signal for the observationsatellite.

In one embodiment, the observation satellite orbits the earth in anorbit having an inclination larger than 175° (175 degrees) and smallerthan 185° (185 degrees). In such an orbit, the observation satellitemoves in particularly advantageous orbit to receive electromagneticenergy transmitted in the direction of the geostationary orbit.

In one embodiment, the observation satellite orbits the earth in anorbit having an apogee differing by no more than 4000 kilometers fromthe geostationary orbit. In this orbit, the observation satellite isable to receive electromagnetic energy that closely corresponds to theactual electromagnetic energy reaching the geostationary orbit. Thistherefore enables the observation satellite to obtain data to provide agood estimation of the actual electromagnetic energy reaching thegeostationary satellites.

In one embodiment, the observation satellite orbits the earth in anorbit having an apogee being any one of: between 31700 and 34700kilometers above mean sea level, and between 36700 and 39700 kilometersabove mean sea level. In this orbit, the collision risk with thegeostationary satellites themselves and other resident space objects(RSOs), including space debris, is further minimized, based on thecurrent public catalog of objects in orbit.

In one embodiment, the observation satellite orbits the earth in anorbit having an eccentricity being smaller than 0.05.

In one embodiment, the observation satellite is not itself acommunication satellite for relaying end user information from one pointof the earth to another point on the earth. Hence, in this embodiment,the observation satellite operates independently from existingcommunication satellites. The observation satellite, as an independententity, also thus provides the capability to monitor spectrum betweenexisting geostationary satellites, in vacant orbital locations along thearc. This allows for the evaluation of spectrum environment in advanceof deploying a satellite asset to a given location, which could informspectrum planning and fleet deployment decisions. This shows that, insome embodiments, the observation satellite can be used beyondinterference management.

In one embodiment, the observation satellite's transmitter is configuredfor transmitting (or retransmitting) using at least one of: (a)downlinking the data (i.e., the data corresponding to above-mentionedcases (i), (ii) or (iii), or any combination thereof) by use of specificdedicated microwave frequencies; (b) downlinking data utilizing atransmitter able to dynamically adjust its transmission frequency band;(c) transmission using low-level spread spectrum; (d) at least oneoptical communication link; and (e) a store and forward method whereinthe transmission (or retransmission) is delayed from the reception ofthe signal.

In one embodiment, the satellite further comprises a receiver connectedto at least one receiving antenna, the receiver having sufficientspectrum agility and capability to be reconfigured to differentfrequencies across a broad range of the radio frequency spectrum. Thus,the satellite can receive electromagnetic energy in a broad frequencyrange as opposed to existing satellites.

The invention also relates to a satellite for obtaining informationabout electromagnetic energy emitted from a source, or from sources, onthe earth. The satellite orbits the earth in an orbit having aninclination larger than 90° (i.e., 90 degrees) and smaller than 270°(i.e., 270 degrees). The satellite comprises means for receiving, whilethe satellite is orbiting relative to the surface of the earth,electromagnetic energy in the radio frequency range, from a source, orfrom sources, on the earth using at least one receiving antenna having areceiving pattern directed towards the earth. The satellite alsocomprises transmitting means for at least one of: (i) retransmitting atleast part of the received electromagnetic energy; (ii) transmittinginformation representing at least part of the received electromagneticenergy; and (iii) transmitting information derived from at least part ofthe received electromagnetic energy.

In one embodiment, a plurality of satellites as described above are usedtogether in order to obtain information about electromagnetic energyemitted from a source, or from sources, on the earth.

The invention also relates to a method for operating a satellite, or aplurality of satellites, as described above.

The invention also relates to a system comprising at least oneobservation satellite according to any one of the preceding embodiments,at least one ground station configured for obtaining, from the at leastone observation satellite, the received electromagnetic energy or theinformation representing, or derived from, the received electromagneticenergy, and at least one processing station. The at least one processingstation is configured for estimating, from the received electromagneticenergy or the information representing, or derived from, the receivedelectromagnetic energy obtained by the at least one ground station, atleast one of: (a) the composition of at least part of the receivedelectromagnetic energy; (b) a location on earth of the origin of atleast part of the received electromagnetic energy; (c) a level ofreceived electromagnetic energy reaching at least part of thegeostationary orbit; and (d) at least one characteristic of an uplinktransmission.

In the above-described system, the processing station is located onearth. The invention is not however limited to such configuration. Asexplained above, the processing may also for example be entirely orpartially carried out in the observation satellite or in anothersatellite.

When more than one observation satellite is employed, the amount ofreceived electromagnetic energy can be increased. Furthermore, theresponse time, i.e. the time from requesting an action from the systemto the corresponding answer, can be reduced.

The invention also relates to a method comprising: obtaining, by atleast one ground station, a signal originating from a satellite (herereferred to as “observation satellite”, as mentioned above) orbiting theearth on an orbit having an inclination larger than 90° (90 degrees) andsmaller than 270° (270 degrees). The signal conveys at least one of: (i)electromagnetic energy received by the observation satellite, (ii)information representing electromagnetic energy received by theobservation satellite, and (iii) information derived fromelectromagnetic energy received by the observation satellite. The methodalso comprises: estimating, by at least one processing station, from atleast part of the received electromagnetic energy or the informationrepresenting, or derived from, at least part of the receivedelectromagnetic energy obtained by the at least one ground station, atleast one of: (a) the composition of at least part of the receivedelectromagnetic energy; (b) a location on earth of the origin of atleast part of the received electromagnetic energy; (c) a level ofreceived electromagnetic energy reaching at least part of thegeostationary orbit; and (d) at least one characteristic of an uplinktransmission.

In the context of the above-described method, the processing station islocated on earth. As already mentioned above however, the invention isnot limited to such configuration. The processing may also for examplebe entirely or partially carried out in the observation satellite or inanother satellite.

In one embodiment, estimating the location comprises estimating alongitude and latitude. In such a manner, the location of the origin(i.e., source) of the electromagnetic energy can be determined.Determination of the source of the electromagnetic energy may thenenable the removal of the interference for example by a satelliteoperator contacting the operator of a transmitting station to stoptransmission or by an operator of a transmitting station effectingnecessary repairs of malfunctioning or misaligned equipment.

In one embodiment, estimating a location on earth of the origin of atleast part of the received electromagnetic energy comprises estimatingthe location of an interference source.

In one embodiment, estimating at least one characteristic of an uplinktransmission comprises estimating an uplink pattern of at least one ofan antenna on earth and a group of antennas on earth.

The invention also relates to a method for operating an observationsatellite, to the use of an observation satellite for theabove-mentioned purposes (i.e., for estimating: (a) the composition ofat least part of the received electromagnetic energy; (b) a location onearth of the origin of at least part of the received electromagneticenergy; (c) a level of received electromagnetic energy reaching at leastpart of the geostationary orbit; and (d) at least one characteristic ofan uplink transmission).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention shall now be described, inconjunction with the appended Figures in which:

FIG. 1 schematically illustrates an exemplary scenario in which asatellite experiences interference;

FIG. 2 schematically illustrates another exemplary scenario in which asatellite experiences interference;

FIG. 3 schematically illustrates an exemplary scenario in whichoperation of a satellite is disturbed by a ground station picking up aterrestrial transmission;

FIG. 4 schematically illustrates an observation satellite in aretrograde orbit and another satellite, in one embodiment of theinvention;

FIG. 5 schematically illustrates an observation satellite in aretrograde orbit and another satellite, in one embodiment of theinvention;

FIG. 6 schematically illustrates a highly inclined satellite, i.e. in aretrograde orbit, in one embodiment of the invention;

FIG. 7 schematically illustrates the composition of an observationsatellite in one embodiment of the invention;

FIG. 8 schematically illustrates the composition of an observationsatellite payload in one embodiment of the invention;

FIG. 9 schematically illustrates a composition of a processing stationin one embodiment of the invention;

FIG. 10 schematically illustrates a transmission from a satellite via arelay;

FIG. 11 schematically illustrates a method in one embodiment of theinvention; and

FIG. 12 schematically illustrates a method in one embodiment of theinvention.

DETAILED DESCRIPTION

The present invention shall now be described in conjunction withspecific embodiments. The specific embodiments serve to provide theskilled person with a better understanding, but are not intended to inany way restrict the scope of the invention, which is defined byappended claims. In particular, the embodiments described independentlythroughout the description can be combined to form further embodimentsto the extent that they are not mutually exclusive.

FIG. 1 schematically illustrates a satellite 30 orbiting the earth on ageostationary orbit (labelled “GEO orbit” on FIG. 1), in order tounderstand problems that some embodiments of the invention address.

Satellite 30 receives an uplink signal (labelled “Uplink” on FIG. 1)from a ground station 60 (for example voice or data to be relayedtowards another point on earth). Additionally, another ground station 50transmits electromagnetic energy (labelled “Interfering carrier” onFIG. 1) towards the location of satellite 30 due to, for example, amisdirected satellite dish (i.e., mis-pointed antenna) or because theradiation pattern of ground station's 50 antenna is not directive enough(for example because its main lobe is too broad, because a side lobe isincidentally in the direction of satellite 30, or because thetransmitting power is too high). The electromagnetic energy transmittedfrom ground station 50 may be intended for an adjacent satellite butpartially reaches satellite 30. The electromagnetic energy transmittedfrom ground station 50 appears as interference for geostationarysatellite 30. The interference may disturb the normal operation of thesatellite 30, for example by reducing the usable capacity of thesatellite link, e.g. in terms of data throughput, useable bandwidth,useable power on a given transponder, noise floor stability, etc.

On FIG. 1, the distance between the satellite's 30 orbit (illustrated bythe dashed line) and the curvature of the earth surface (illustrated bythe plain line) are schematic and not to scale. Likewise, the depictedground stations and the satellite are not to scale either. These remarksapply likewise to FIGS. 2, 3, 4, 5, 6, and 10.

FIG. 2 schematically illustrates another scenario in which the satellite30 experiences interference, in order to understand problems that someembodiments of the invention address.

In this case, a ground station 60 transmits an uplink signal to asatellite 31 travelling in a lower orbit than satellite 30. For example,satellite 31 is in a low earth orbit (LEO) or a medium earth orbit (MEO)and satellite 30 is in a geostationary orbit. Although the uplink signalis not intended for satellite 30, the electromagnetic energy carryingthe uplink signal reaches satellite 30 since, at least at one point intime, satellites 30, 31 are in the same line of sight seen from groundstation 60. Therefore, satellite 30 may experience an interference. Thesame problem may occur when satellite 31 is travelling in a higher orbitcompared to satellite 30. Such a situation is for example discussed inThomas J. Lang, “Conjunction/Interference Between LEO and GEO Comsats”,Proceedings of the AAS/AIAA Astrodynamics Specialist Conference held inSun Valley, Id., 4-7 August 1997, AAS Paper 97-668.

FIG. 3 schematically illustrates another scenario in which operation ofa satellite may be disturbed, in order to further understand problemsthat some embodiments of the invention address.

A base station 61, for example used in a mobile communication system,broadcasts a signal to mobile devices or other base stations. Thistransmission is incidentally captured by a ground station 60. In otherwords, ground station 60 unintentionally includes the transmission in anuplink sent to satellite 30. Hence, satellite 30 receiveselectromagnetic energy which not only comprises an uplink signal forsatellite 30 but also a part not intended for satellite 30.

The occurrence of this type of interference phenomenon has beenrecognized by spectrogram analysis. Indeed, it has been shown in thepast that, for example, a GSM rebroadcast may occur as evidenced by aspectrogram analysis clearly showing the timing correction bursts. InGSM indeed, a timing burst is sent after every tenth frame. Since eachGSM frame is 4.615 ms, the timing burst has been seen every 46.15 ms.This situation may occur in some circumstances for example if a mobilecommunication cell tower (base station) is close to a VSAT uplinkantenna (such as a few hundred meters from each other). The VSAT uplinkantenna is then susceptible of capturing and retransmitting signals fromthe mobile communication cell tower towards the geostationary orbit.

Now that some exemplary interference scenarios have been explained withreference to FIGS. 1 to 3, embodiments of the invention will bedescribed in more detail.

In one embodiment, an observation satellite 10 is used to obtaininformation on the electromagnetic energy reaching a satellite 30 (whichis for example a geostationary satellite) or, more generally, reachingan orbit or group of orbits. In that respect, FIG. 4 schematicallyillustrates an observation satellite 10 in a retrograde orbit (althoughseveral observations satellites 10 may also be used), a ground station20 receiving transmissions from and transmitting control signals toobservation satellite 10, a processing station 40 processing thereceived transmissions, a satellite 30 in a geostationary orbit, atransmission station 60 transmitting an uplink signal towards satellite30 and a transmission station 50 incidentally transmittingelectromagnetic energy towards satellite 30. In the example shown inFIG. 4, it is assumed that satellite 30 is in a geostationary orbit,although the invention is not limited to this scenario.

Observation satellite 10, which orbits the earth in a retrograde orbit,is able to receive the transmissions from ground stations 50, 60.Further, observation satellite 10 sends information about the receivedelectromagnetic energy to a ground station 20. The information is thenused in a processing station 40 for processing, for example to derivevarious kinds of information. For example, processing station 40 maydetermine the location of interfering ground station 50 so as to be ableto adjust the satellite dish of ground station 50 or to be able to adaptthe receiving pattern of the antenna of satellite 30 to avoid receivingthe undesirable signals from ground station 50 (for example bygenerating a null in the direction of interfering ground station 50).

FIG. 5 schematically illustrates a configuration in which observationsatellite 10 orbits at an altitude greater than the geostationary orbit,rather than lower than the geostationary orbit. Besides, theconfiguration illustrated by FIG. 5 is similar to the one illustrated byFIG. 4.

The advantages of using a retrograde orbit for observation satellite 10will now be explained in more detail.

Retrograde Orbit

To achieve maximum value of a satellite with spectrum sensingcapabilities (i.e., an observation satellite), some embodiments of thepresent invention extends sensing coverage to as many satellites aspossible. Typically the motion of a satellite relative to othersatellites in a given orbit requires expending fuel, resulting in ashortened lifespan, increased spacecraft cost, or that the speed ofrelative drift be slow. Theoretically one could orbit very quicklyrelative to other satellites at the appropriate altitude if thesatellite were to expend fuel continuously. However, this is notpractical and probably not even feasible with current technology.Therefore, the possible drift rate, while also maintaining theadvantageous vantage point altitude, would be at such a slow rate as toextremely limit the value of the system. While a slow drift rate may fitthe more strategic value-added needs, since fleet deployment andspectrum development operate on long time scales, a slow drift rate alsoprecludes it as an operational monitoring platform suitable forresponsiveness to active communications transmissions since it couldtake months to re-deploy the asset to the needed orbital location. Forthis reason, and to maximize the number of satellites served, theproposed solution is for observation satellite 10 to operate in analternative retrograde orbit at near geo-synchronous altitude, orbitingthe Earth counter to the natural rotation about its axis. A retrogradeorbit is here understood as being an orbit having an inclination largerthan 90° and smaller than 270°.

The use of satellites orbiting on a retrograde orbit is known, but forother applications and configurations. For example, US 2008/0081556 A1relates to placing a satellite in a retrograde orbit for observing andinspecting satellites.

Preferably, the inclination of the retrograde orbit is between 175° and185° such that the observation satellite 10 moves close to theequatorial plane. An additional schematic illustration of a retrogradeorbit is shown in FIG. 6. This orbit has the advantage of continuous andnatural motion relative to geosynchronous satellites 30, and provides areasonable frequency of flyby for each geosynchronous spacecraft ofinterest. The retrograde orbit allows measurement at the appropriatealtitude without the need for expending significant fuel. Morespecifically, the retrograde orbit enables the desired revisit rate andvantage point as natural motion such that the only fuel necessary isthat which is required to achieve and maintain the orbit for thelifetime of the satellite (similar to all other satellites). Thealtitude is preferably similar to geostationary satellites 30, plus orminus up to several thousand kilometers in order to provide a buffer ofsafety for conjunction with other satellites at that altitude. Thealtitude is significant to the sensing operations of the observationsatellite 10 because being at a similar altitude allows the observationsatellite 10 to monitor the same transmissions—or almost the sametransmissions—that the geostationary satellites 30 receive. If thespacecraft's altitude was too high or too low, it would potentially misssome transmission signals (from antennas with low elevation angles onearth) and it would potentially receive unwanted microwave energy.

In case the altitude of the observation satellite 10 is approximatelythe same as geostationary satellites 30, i.e. the observation satellite10 for example orbits the earth in an orbit having an altitude differingby no more than 4000 kilometers from the geostationary orbit, theorbital period remains approximately one satellite rotation everytwenty-four hours. However, since the earth's rotation and geostationarysatellite's orbit would be equal in magnitude but opposite in direction,the observation satellite 10 passes each geostationary satellite 30roughly twice per twenty-four hours. The orbit can be also described asa prograde geosynchronous type with an inclination of 180 degrees. Therapid frequency of arrival at each geostationary satellite 30 andextensive coverage across the entire arc thus makes a single observationsatellite 10, with its microwave sensing capabilities, capable ofserving an entire fleet.

Since a retrograde orbit is not useful in a traditional geosynchronoussatellite communications sense, few retrograde launches have beenperformed. Additional energy is required to launch a retrogradesatellite from the surface of the Earth since one has to reverse themomentum provided by the natural rotation of the Earth to achieve anorbit in the opposite direction.

The most straightforward launch approach is to launch westward from thesurface of the earth, accounting for additional thrust to achieve thenecessary delta-v in the retrograde direction. However, since mostlaunches are prograde, launch sites are located for favorable eastwardlaunch conditions (a large body of water directly to the east). Anadditional approach is a more traditional eastward launch into asuper-synchronous transfer orbit and utilizes propulsion on board thespacecraft to increase the inclination to 180 degrees over time. Suchlaunches however should not be considered as the only means forachieving such an orbit.

Another launch approach is to use the moon to perform a lunar swing-byand ‘reverse’ direction relative to the rotation of the earth, asdiscussed for example in Aravind, R., et al, “Mission to RetrogradeGeo-equatorial Orbit (RGEO) using lunar swing-by”, 2012 IEEE AerospaceConference (3-10 March 2012), pp. 1-8.

Satellite Composition

FIG. 7 schematically illustrates the composition of an observationsatellite 10 in one embodiment of the invention. The observationsatellite 10 and its equipment may be built on top of an existingspacecraft bus, including the power, propulsion, control, thermal, andother subsystems. The specialized payload (i.e., equipment) couldcomprise the following major subsystem components: receive-only arrays11 and controlling components 12 (control unit) necessary for enablingspace-based geolocation, spectrum processing and analysis hardware, acommunications subsystem 13 (labeled “COM” on FIG. 7) to receivecommands from the ground and download processed data (temporarilystoring data for transmission if necessary) and telemetry, and apropulsion system 14. The propulsion subsystem is depicted specificallydue to the significance of this subsystem in achieving the prescribedorbit, depending on the method of launch. However, the observationsatellite 10 may not contain all the above mentioned subsystems andcould contain additional subsystems in other embodiments.

The antenna 11 has a receiving pattern directed towards the earth and issuitable for receiving electromagnetic energy in the radio frequencyrange. The received electromagnetic transmission may comprise anintentionally transmitted signal and/or noise-like transmissions.Preferably, the antenna 11 is suitable for receiving transmissions in arange used by communication satellites, e.g. L-band (1 to 2 GHz), S-band(2 to 4 GHz), C-band (4 to 8 GHz), X-band (8 to 12 GHz), K_(u)-band (12to 18 GHz), and K_(a)-band (26.5 to 40 GHz). The antenna 11 may besuitable for receiving electromagnetic energy with any types ofpolarization (e.g., linear, vertical, horizontal, elliptical andcircular) or only some types of polarization. Furthermore, the receivingpattern is directed towards the earth when the observation satellite 10is correctly positioned, i.e. the observation satellite 10 stably orbitsthe earth.

Furthermore, the control unit 12 may obtain information about themodulation scheme of the received electromagnetic energy (e.g.phase-shift keying, quadrature amplitude modulation, time divisionmultiple access, code division multiple access, frequency divisionmultiple access).

For communications between the observation satellite 10 and the ground,including command carrier, telemetry and data download link, one of thebelow techniques may for example be used:

-   -   (a) Downlinking the data by use of specific dedicated microwave        frequencies. Typically the data downlink frequencies are        different than those normally used by communications satellites        30 being analysed in order to avoid causing interference.    -   (b) Downlinking data utilizing an RF transmitter able to        dynamically adjust its transmission frequency band so as to        avoid interference to nearby satellites as the observation        satellite 10 moves. This could be a pre-scheduled sequence or        autonomous.    -   (c) Transmission using low-level spread spectrum. Utilizing        spectrum which is (or is not) already in use and transmitting a        very low level signal. The data rate is greatly reduced per MHz,        but it results in a negative carrier to noise (C/N) ratio and        thus has minimal effect on any traffic being transmitted at the        same frequencies.    -   (d) An optical channel. In this case, more than one ground        station 20 may be needed to ensure stable downlink communication        since clouds and other weather phenomena may interrupt the        optical link.    -   (e) A store and forward method, using any of the above methods        but storing data from other parts of the orbit and downlinking        later in a different part of the orbit (for example at a place        with fewer frequency ranges already in use and high data        throughput could be achieved).

Regarding the downlink communication from the observation satellite 10,any above methods may be used by:

-   -   (1) ‘Wide’ area transmission, covering a part of or the entire        earth from the satellite's vantage point.    -   (2) Small beam tracking antennas on-board which track a number        of ground stations, thus maximizing power/throughput, as well as        streamlining coordination and enhancing data-security.    -   (3) A transmission link to another satellite acting as a data        relay.

The third alternative is schematically illustrated in FIG. 10.Observation satellite 10 uses a relay satellite 31 to send a downlinktransmission to ground station 20. More specifically, observationsatellite 10 transmits towards satellite 31. The relay satellite 31 thenrelays the transmission towards ground station 20, possibly after someprocessing. Observation satellite 10 may transmit, to relay satellite31, through an optical link (i.e., optical satellite-to-satellitetransmission).

FIG. 8 schematically illustrates the composition of an observationsatellite 10 in one embodiment of the present invention. In thisembodiment, the observation satellite 10 receives electromagnetic energyin the C and K_(u) band using a phased array antenna 11. The receivedelectromagnetic energy is then converted to a common frequency suitablefor the digital signal processor. An A/D converter 121 digitalizes theanalog signal. Fast-Fourier processing is performed on the signal fromthe A/D converter 121 in a FFT unit 122. A processing unit 123 mayfurther process the output from the FFT unit 122 before it is stored ina buffer 131. The data stored in the buffer is transmitted to a groundstation 20 via a downlink antenna 132.

The invention is however not limited to the exemplary implementationillustrated by FIG. 8. FIG. 8 illustrates a simple implementation andonly serves to exemplify a functional hardware configuration for oneapproach. Many other possible implementations may be used.

Space-Based Geolocation

Space-based geolocation means to determine a location on the surface ofthe earth by means of an object in space. For doing so, many possibledirection finding (DF) techniques are possible. One possible mechanicalimplementation may, for example, involve rotating the spacecraft untilthe transmission signal is at its strongest, so that the orientation ofthe spacecraft is parallel to the transmitted signal vector. Moresophisticated approaches have more complex electronic and/or mechanicalsteps. Some additional approaches are described below.

From the natural vantage point in space, observation satellite 10 canobserve and monitor nearly all uplink transmissions to geostationarysatellites and gather relevant information regarding the directivity ofuplink signals. The directivity of incoming uplink signals to ageostationary satellite is typically difficult to ascertain from theground after the signal has been received and retransmitted by acommunications satellite.

In orbit, determination of uplink locations can be achieved in a morestraightforward manner compared to ground-based geolocation by directlyresolving the incident angle of the transmission to the spacecraft and,given the spacecraft attitude and position, thus the location on thesurface of the earth. This approach has significant advantages to theground-based approach because it is not subject to the limitation ofrelying on external sources of data and is a more robust determinationmethod since it measures uplink energy directly, rather than relying onfrequency and temporal Doppler shift from the downlink signals ofadjacent satellites. This space-based geolocation capability allows forthe localization of unauthorized uplink locations transmitting to aspacecraft and causing interference to contracted traffic andcontaminating spacecraft capacity.

Performing space-based geolocation can be performed with a multitude oftechniques and hardware solutions. Four main technique classificationsmay for example be considered, namely: (T1) Direction finding (DF)utilizing ‘nulling’ of uplink sources by means of a flexible orsteerable antenna pattern, (T2) Frequency of Arrival (FoA) or Doppleranalysis by examining the frequency offset or change in frequency of anincident signal, (T3) Angle of Arrival (AoA) analysis by comparing thereceived signal (phase, amplitude, etc.) between two or more antennaelements, and (T4) Spectral estimation by correlation of impingingsignals across elements in a multi-element array.

(T1) DF techniques generally utilize the gain characteristics of anantenna to determine a bearing to the impinging signal. By variation ofthe gain of an antenna in a given direction, either through mechanicalmovement or electrical means, the characteristics of a transmittedsignal as received by the antenna may also vary, thus providing someunderstanding of the nature of the source of the transmitted signal.Variation of gain in a given direction could be accomplished forexample, by a relative motion of the antenna with respect to the sourceof a transmitted signal by means of a gimble or natural motion, or by adynamic variation of the gain pattern of an antenna by means of anelectronically steerable antenna array. DF techniques are mature andmany hardware implementations have been created to apply thesetechniques for various applications.

In one instance, DF techniques could be applied for space-basedgeolocation of uplink transmissions by equipping an observationsatellite 10 with an antenna or antennas with a high gain slopecharacteristic and which cover a portion of the earth's surface as seenby the observation satellite 10. At any given point in orbit, theantenna patterns of the observation satellite 10 could be mechanicallyor physically moved around the surface of the earth to ‘seek’ theinterfering carrier. This motion could be deliberately controlled orcould process in a repeating pattern to provide coverage of the entireearth. Continuous monitoring of the signal levels in comparison to areference level as the pattern shifts provides exact measurements as thefootprint approached and moved away from the target signal. Withsufficient knowledge of the antenna patterns themselves, the motion ofantenna patterns and the spacecraft attitude, the vector towards theinterfering signal source can be determined and the origin on thesurface of the earth computed. Since this technique relies on directenergy measurement (rather measurement of the lack of energy while thetransmitter entering or leaving a pattern ‘null’), such a system wouldbe robust and applied to a wide range of uplink signals.

Among the most flexible hardware solutions is to use active phasedantenna arrays to electronically steer the footprint pattern on thesurface of the earth. This has the advantage of no mechanically movingparts, and be adaptable to perform several types of searching functions(repeating pattern, dedicated seek, etc.). Other types of hardwareimplementations for the third proposed technique as described aboveinclude small spot beam antennas or antenna clusters which aremechanically moved or rotated on-board the spacecraft, or fixed antennasor antenna clusters with spacecraft nutation in roll, pitch, yaw or somecombination thereof.

(T2) Frequency of Arrival or Doppler analyses could also be employed forperforming space-based geolocation. Due to relative motion between theobservation satellite 10 and a transmitting station 50, the frequency ofthe signal received at an antenna on the observation satellite 10 may beoffset to the true frequency as transmitted by transmitting station 50.In addition, because of the orbital geometry the rate of change of theDoppler induced offset may also be changing. By comparing the frequencyoffset to a known reference, or by comparing the frequency offsetbetween two elements onboard the spacecraft, or by analyzing the rate ofchange of the frequency offset as measured by the observation satellite,an understanding of the possible originating locations on the surface ofthe earth can be derived. The frequency offset may be particularlysignificant to observe due to the high relative velocity of theobservation satellite 10 in a retrograde orbit.

For performing this methodology, the observation satellite 10 wouldrequire at least one receiving antenna, but may have more than one.Since this approach relies largely on analysis of frequency of thetransmitted signal, there are few requirements of the onboard antennapatterns, gain and steer-ability. In addition, it is worth noting thatthis technique for space-based geolocation is dependent upon a stabletransmission frequency or known variations in transmission frequencysuch that variations in the true frequency are not perceived as Dopplervariations due to relative motion.

(T3) Space-based geolocation by angle of arrival analysis could utilizea plurality of antennas onboard the spacecraft with overlapping antennapatterns on the surface of the earth. Reference measurements between twoantenna elements, as a simple example, may be one way to gain basicinformation about the angle of arrival of a signal. Consider twoantennas, with overlapping but not identical reception patterns, suchthat the same signal received by the two antennas is receiveddifferently and at a different power level. Through knowledge of thereception characteristics of both antenna elements, a crudeunderstanding of the possible direction of arrival can be determinedbecause only a small portion of possible direction vectors would resultin the observed signals for both antennas. This process can be refinedand improved with more complex and a greater number of antennas to auseable degree.

By measuring amplitude and/or (more commonly) phase of a given signal byone or more of elements and comparing the differential in ratio to eachother or to a reference measurement, an understanding of the incidentangle of the source onto the antenna can be determined and the origin ofthe transmission sources can be derived. In this case, high gain, highslope and poor roll-off performance of the antenna functions areadvantageous in achieving a high resolution and the measurementaccuracy. The technique is relatively simple to implement and mayprovide robust measurements for a wide range of uplink signal types.

(T4) Spectral estimation techniques rely on measurement of signalsources impinging on an observation satellite 10 by multiple elements inan array. As a result, spectral estimation direction of arrivaltechniques require multiple antenna elements configured in asophisticated array onboard an observation satellite 10 and pointingtoward the earth, such that the antenna gain patterns overlap and theelements are spaced appropriately according to the intended signal ofinterest.

Measurements of the amplitude and phase of incident signals at a givenfrequency across each element can be correlated to produce an estimateof the composition of the incident signals, namely the angle ofincidence on the array. Many measurements may be taken sequentially tostatistically improve the characterization of a signal throughaveraging. Mathematically intensive processing can further improveperformance through the use of sub-space methods, such as MultipleSignal Classification (MUSIC). Though computationally expensive andrequiring a sufficiently large antenna array, some correlationapproaches can provide high-fidelity estimates of the direction ofarrival of a plurality of incident signals simultaneously. Sufficientknowledge of the antenna array and orientation and position of theobservation satellite 10, space-based geolocation can be performed asthe origin of each incident signal on the surface of the Earth can bederived.

The optimal solution may in fact incorporate multiple of the abovetechniques and indeed some techniques may be compatible to perform theanalyses using the same spacecraft hardware. Synthesis of the output ofmultiple techniques as well as synthesis of sequential analyses as theobservation satellite 10 moves and the incident angle of a given signalchanges may produce superior performance.

With the exception of some Doppler offset measurements in technique (T2or T3), it is worth noting that the space-based geolocation is notdependent on the retrograde (high relative velocity) motion of thespacecraft; but rather that the retrograde motion enables relativelyrapid response time and coverage to perform space-based geolocation foreach satellite orbiting in a prograde direction at a given altitude.

In one embodiment, which extends in a sense the concept of cooperationbetween a plurality of satellites as illustrated in FIG. 10, afractionated spacecraft or multiple spacecraft working in tandem areused. For example, in one embodiment, a system involving a plurality ofobservation satellites is used. This may enable the use of smallerspacecraft each having at least one antenna element taking measurementssimultaneously at some distance, and then communicating those signals tothe larger spacecraft. The spacecraft may also be identically sized. Itmay be advantageous for performing space-based geolocation to haveantennas which are spaced very far apart (i.e. synthetic apertureinterferometry) and they need not be on the same spacecraft.

Ground-Based Geolocation with an Observation Satellite

In one embodiment, traditional ground-based time difference of arrival(TDOA) processing or frequency difference of arrival (FDOA) processingis used as a geolocation technique involving the observation satelliteor satellites. Namely, the observation satellite acts as a secondarysatellite to the target satellite for TDOA-FDOA measurements or twoobservation satellites in proximity may be used to perform ground-basedTDOA-FDOA analysis of the signals. This involves transmitting the datacorresponding to above-mentioned cases (i), (ii) or (iii), or anycombination thereof (i.e., as discussed above, at least part of thereceived electromagnetic energy or the information representing, orderived from, at least part of the received electromagnetic energy) tothe ground from the observation satellite(s).

This technique is identical to existing ground-based geolocationtechniques, except that the technique involves one or more observationsatellites in retrograde orbits which are specifically intended forassisting in signal analysis and determination of the origin of uplinksignals. In that sense, the technique might also be called “space-based,ground-based geolocation”.

System Composition

In addition to observation satellite 10, a system in one embodiment ofthe invention may include ground-based infrastructure and communicationsto relay processed data to the ground. Several ground stations 20 aroundthe earth with tracking antennas would perform telemetry, tracking andcommunication (TT&C) functions as well as receive transmitted data fromobservation satellite 10. Horizon to horizon time for observationsatellite 10 at an altitude near the geostationary orbit isapproximately four hours, which is within a reasonable speed forexisting antenna systems. At least three or four ground stations 20 arerequired to maintain constant communication with observation satellite10. In addition to traditional spacecraft commanding and control, groundstations 20 also send any necessary payload commands or queries toobservation satellite 10, directing the sensing operations to producespecific measurements as desired by users.

The data downloaded by observation satellite 10 may be large, and thereception and data storage chain on the ground is sized appropriately tohandle a continuous flow of data from observation satellite 10 through atraditional demodulation and decryption reception chain. Data may bestored in one or more processing stations 40, fed from each groundstation 20 sequentially as the satellite reception area changes.Depending on processing implementation on-board and the geolocationtechnique employed, an additional layer of processing would be performedat the centralized data center in order to produce the desired outputand data format. Front end interfaces may allow users to direct payloadcommands to the observation satellite 10 through the ground stations 20as well as allow access to the received and processed data.

A processing station 40 in one embodiment of the invention isschematically illustrated in FIG. 9. As shown on FIG. 4, processingstation 40 is located on earth. However, as explained above, since someprocessing may additionally or alternatively be performed within theobservation satellite 10, or within another satellite, the samefunctions and/or units may also be incorporated, to the extent that thisis necessary, within the observation satellite 10, or within anothersatellite.

Processing station 40 comprises a processing unit 41, a memory unit 42coupled to the processing unit 41, and a communication unit 43 coupledto the processing unit 41.

Processing unit 41 may include a processor, a microprocessor, orprocessing logic that may interpret and execute instructions, as thosedescribed in the present document (for instance with reference to FIG.11). These operations may be performed in response to processing unit 41executing software instructions contained in a computer-readable medium,such as memory unit 42. The software instructions contained in memoryunit 42 may cause processing unit 41 to perform operations or processesdescribed herein. Alternatively, hardwired circuitry may be used inplace of or in combination with software instructions to implementprocesses and/or operations described herein. Thus, implementationsdescribed herein are not limited to any specific combination of hardwareand software.

Memory unit 42 is configured to store transmissions from one or moreobservation satellites 10. Memory unit 42 may include a RAM or anothertype of dynamic storage device that may store information andinstructions for execution by processing unit 41. Memory unit 42 mayalso include a ROM device or another type of static storage device thatmay store static information and instructions for use by processing unit41. Memory unit 42 may also include a magnetic and/or optical recordingmedium and its corresponding drive.

Communication unit 43 is configured to communicate with one or moreground stations 20. Communication unit 43 may include anytransceiver-like mechanism that enables processing station 40 tocommunicate with other devices and/or systems.

Memory unit 42 may store a computer program loadable into the processingunit 41 comprising code for executing the steps of the described methodembodiments according to the present invention. Although notillustrated/for the sake of conciseness, a bus—including a path thatpermits communication among the components of processing station 40—, aninput device—including a mechanism that permits an operator to inputinformation to processing station 40, such as a keypad, a keyboard, amouse, a pen, voice recognition and/or biometric mechanisms, etc.—and anoutput device—including a mechanism that outputs information to theoperator, such as a display, a printer, a speaker, etc.—may also beincluded in processing station 40.

FIG. 11 is a flowchart of a method performed by the system in oneembodiment. In step S11, at least one ground station 20 obtains a signaloriginating from an observation satellite 10 orbiting having aninclination larger than 90° and smaller than 270° (i.e., in a retrogradeorbit), and conveying at least one of: (i) electromagnetic energyreceived by the satellite, information representing (ii) electromagneticenergy received by the satellite, and (iii) information derived fromelectromagnetic energy received by the satellite.

The information representing the received electromagnetic energy may beproduced in observation satellite 10 by digitalizing and compressing thereceived electromagnetic energy. In the last alternative, theinformation derived from the received electromagnetic energy may beproduced in the observation satellite 10 by determining the location(e.g. longitude and latitude) of the source of the receivedelectromagnetic energy.

In step S12, at least one processing station 40 estimates from thereceived electromagnetic energy or the information representing, orderived from, the received electromagnetic energy obtained by the atleast one ground station, at least one of: (a) the composition of atleast part of the received electromagnetic energy; (b) a location onearth of the origin of at least part of the received electromagneticenergy; (c) a level of received electromagnetic energy reaching at leastpart of the geostationary orbit; and (d) at least one characteristic ofan uplink transmission. In one embodiment, the ground station 20 and theprocessing station 40 may be collocated or form a single station.

The composition of at least part of the received electromagnetic energymay comprise the energy spectrum or polarization of the receivedelectromagnetic energy. The location on earth of the origin of at leastpart of the received electromagnetic energy may comprise the longitudeand latitude of the origin. The level of received electromagnetic energyreaching at least part of the geostationary orbit is useful fordetermining an interference map of the geostationary orbit. Acharacteristic of an uplink transmission may comprise the modulationscheme of the uplink transmission.

FIG. 12 is a flowchart of a method performed by a system in oneembodiment of the invention.

In steps S21 and S22, immediate or preloaded configuration commands areexecuted and the satellite configuration is loaded. In step S23, thesatellite receives uplink electromagnetic energy. The uplinkelectromagnetic energy may, for example, include energy intended forother satellites. In step S24, the received electromagnetic energy issubjected to a frequency conversion or frequency translation. Additionalsteps may include analogue to digital conversion, digital processing ofreceived electromagnetic energy, data reduction or fusion of multiplesources of data, digital to analogue conversion, frequency translation,etc.

In step S25, at least part of the received electromagnetic energy istransmitted (for example towards a ground station on earth). Thereceived electromagnetic energy is digitized in step S26 and digitallyprocessed in step S27. The processing may include utilizing knownprocessing functions such as fast Fourier transformation, Doppler shiftanalysis, Doppler rate analysis, direction-of-arrival orangle-of-arrival processing, time difference of arrival processing,frequency difference of arrival processing, and/or power, frequency,phase reference comparison between two or more receiving elements orinstances in time.

The output of step S27 may be used for various processing steps. In stepS28, the processed data may be synthesized with external sources such asthe characteristics and origin of reference signals, orbital position,velocity and orientation data of the satellite. The synthesized data maythen be used in coordinate transformation and projection of anapproximate location on the surface of the earth in step S29. Thereby,the location of a transmitter can be estimated in step S30(geolocation). Alternatively or additionally, the synthesized data maybe synthesized using multiple analyses over time in step S31. Thereby,antenna characterizations, compliance measurements or spectralenvironment trending may be obtained in step S32.

Furthermore, the output of step S27 may be further processed bydemodulating the received electromagnetic energy in step S33. Thereby,it becomes possible to perform a carrier signal analysis (step S34).

In further embodiments of the invention, any one of the above-describedprocedures, steps or processes may be implemented usingcomputer-executable instructions, for example in the form ofcomputer-executable procedures, methods or the like, in any kind ofcomputer languages, and/or in the form of embedded software on firmware,integrated circuits or the like.

Although the present invention has been described on the basis ofdetailed examples, the detailed examples only serve to provide theskilled person with a better understanding, and are not intended tolimit the scope of the invention. The scope of the invention is muchrather defined by the appended claims.

In any one of the above embodiments, the observation satellite may alsocomprise an additional receiving antenna having a receiving patterndirected away from the Earth, the antenna being suitable for receivingelectromagnetic energy in the radio frequency range. In such a manner,it is possible for the observation satellite to receive a given signalbeing inputted to a given target satellite (orbiting at an altitudegreater than the orbit in which the observation satellite orbits), aswell as the repeated given signal being outputted by the targetsatellite. For example, the target satellite may be orbiting in ageostationary orbit, in which case the observation satellite'sadditional receiving antenna may be oriented in or near the zenithdirection—provided that the observation satellite orbits at an altitudesmaller than the geostationary orbit. The advantage of this approach isthat it enables using the input and output signals together to cancelout some sources of error (any slight frequency drift of the givensignal, for instance, which otherwise would result in geolocationsolution errors) and enables characterization of performance of thetarget satellite itself.

The invention claimed is:
 1. A satellite for obtaining information aboutelectromagnetic energy emitted from a source, or from sources, on theearth, the satellite orbiting the earth in an orbit having aninclination larger than 90° and smaller than 270°; and the satellitecomprising: at least one receiving antenna, the at least one receivingantenna having a receiving pattern directed towards the earth, andsuitable for receiving electromagnetic energy in the radio frequencyrange as the satellite is orbiting relative to the surface of the earth,and a transmitter configured for at least one of: retransmitting, toanother spacecraft, hereinafter referred to as “relay spacecraft”, atleast part of the received electromagnetic energy, transmitting, to therelay spacecraft, information representing at least part of the receivedelectromagnetic energy; and transmitting, to the relay spacecraft,information derived from at least part of the received electromagneticenergy.
 2. The satellite of claim 1, wherein the relay spacecraft towhich the transmitter is retransmitting and/or transmitting is a relaysatellite.
 3. The satellite of claim 1, being suitable for obtaininginformation about electromagnetic energy emitted from a source, or fromsources, on the earth and reaching the geostationary orbit.
 4. Thesatellite of claim 1, wherein the transmitter is configured to transmit,towards the earth through the relay spacecraft, at least part of thereceived electromagnetic energy, or the information representing, orderived from, at least part of the received electromagnetic energy. 5.The satellite of claim 1, wherein the information derived from at leastpart of the received electromagnetic energy is obtained by processing atleast part of the received electromagnetic energy within the satellite.6. The satellite of claim 5, wherein the processing comprises at leastone of: selectable down-conversion of analog signal to commonintermediate frequency; analog-to-digital conversion of signals providedby at least part of the received electromagnetic energy; spectrumanalysis of at least part of the received electromagnetic energy;Doppler shift analysis of at least part of the received electromagneticenergy; Doppler rate analysis of at least part of the receivedelectromagnetic energy; direction of arrival or angle of arrivalprocessing; time difference of arrival (TDOA) processing; frequencydifference of arrival (FDOA) processing; reference measurements betweentwo or more antenna elements; data filtering; and data compression. 7.The satellite of claim 1, wherein the at least one receiving antenna issuitable to receive electromagnetic energy in a radio frequency rangebetween 1 GHz and 100 GHz.
 8. The satellite of claim 7, wherein the atleast one receiving antenna is suitable to receive electromagneticenergy in a radio frequency range being at least one of: between 1 and 2GHz; between 2 and 4 GHz; between 4 and 8 GHz; between 8 and 12 GHz;between 12 and 18 GHz; and between 26.5 and 40 GHz.
 9. The satellite ofclaim 1, wherein the at least one receiving antenna is suitable toreceive electromagnetic energy in a radio frequency range used bygeostationary satellites to receive, or send, signals from, or to, theearth.
 10. The satellite of claim 1, wherein the at least one receivingantenna is suitable to receive electromagnetic energy having at leastone of: a linear polarization; a vertical polarization; a horizontalpolarization; an elliptical polarization; and a circular polarization.11. The satellite of claim 1, wherein the at least one receiving antennais configured to receive, during one orbital period, electromagneticenergy from an area covering more than half of the surface of the earth.12. The satellite of claim 1, wherein the received electromagneticenergy comprises more than only information intended for controlling thesatellite itself.
 13. The satellite of claim 1, wherein the receivedelectromagnetic energy comprises energy intended for at least one othersatellite.
 14. The satellite of claim 1, wherein the satellite orbitsthe earth in an orbit having an inclination larger than 175° and smallerthan 185°.
 15. The satellite of claim 1, wherein the satellite orbitsthe earth in an orbit having an apogee differing by no more than 4000kilometers from the geostationary orbit.
 16. The satellite of claim 15,wherein the satellite orbits the earth in an orbit having an apogeebeing any one of: between 31700 and 34700 kilometers above mean sealevel; and between 36700 and 39700 kilometers above mean sea level. 17.The satellite of claim 1, wherein the satellite orbits the earth in anorbit having an eccentricity being smaller than 0.05.
 18. The satelliteof claim 1, wherein the satellite is not itself a communicationsatellite for relaying end user information from one point of the earthto another point on the earth.
 19. The satellite of claim 1, wherein thetransmitter is configured for transmitting, to the relay spacecraft, atleast part of the received electromagnetic energy or the informationrepresenting, or derived from, at least part of the receivedelectromagnetic energy using at least one optical communication link.20. The satellite of claim 1, wherein the transmitter is configured fortransmitting, to the relay spacecraft, at least part of the receivedelectromagnetic energy or the information representing, or derived from,at least part of the received electromagnetic energy using at least oneof: (a) specific dedicated microwave frequencies; (b) a transmitter ableto dynamically adjust its transmission frequency band; (c) low-levelspread spectrum; and (d) a store and forward method.
 21. The satelliteof claim 1, wherein the satellite further comprises a receiver connectedto at least one receiving antenna, the receiver having sufficientspectrum agility and capability to be reconfigured to differentfrequencies across a broad range of the radio frequency spectrum. 22.The satellite for obtaining information about electromagnetic energyemitted from a source, or from sources, on the earth, the satelliteorbiting the earth in an orbit having an inclination larger than 90° andsmaller than 270°; and the satellite comprising: receiving means forreceiving, as the satellite is orbiting relative to the surface of theearth, electromagnetic energy in the radio frequency range, from asource, or from sources, on the earth using at least one receivingantenna having a receiving pattern directed towards the earth, andtransmitting means for at least one of: retransmitting, to anotherspacecraft, hereinafter referred to as “relay spacecraft”, at least partof the received electromagnetic energy, transmitting, to the relayspacecraft, information representing at least part of the receivedelectromagnetic energy; and transmitting, to the relay spacecraft,information derived from at least part of the received electromagneticenergy.
 23. The satellite of claim 1, wherein the satellite furthercomprises an additional receiving antenna, wherein the additionalreceiving antenna has a receiving pattern directed away from the Earth,and is suitable for receiving electromagnetic energy in the radiofrequency range.
 24. A system comprising at least two satellitesaccording to claim
 1. 25. A system comprising: at least one satelliteaccording to claim 1, and the relay spacecraft.
 26. The system of claim25, further comprising: at least one ground station configured forobtaining, from the relay spacecraft, the received electromagneticenergy or the information representing, or derived from, the receivedelectromagnetic energy, and at least one processing station configuredfor estimating, from the received electromagnetic energy or theinformation representing, or derived from, the received electromagneticenergy obtained by the at least one ground station, at least one of: thecomposition of at least part of the received electromagnetic energy; alocation on earth of the origin of at least part of the receivedelectromagnetic energy; a level of received electromagnetic energyreaching at least part of the geostationary orbit; and at least onecharacteristic of an uplink transmission.
 27. A method for operating asatellite according to claim
 1. 28. A method comprising: obtaining, byat least one ground station from a relay spacecraft, a signaloriginating from a satellite orbiting the earth on an orbit having aninclination larger than 90° and smaller than 270°, and conveying atleast one of: electromagnetic energy received by the satellite,information representing electromagnetic energy received by thesatellite, and information derived from electromagnetic energy receivedby the satellite, and estimating, by at least one processing station,from at least part of the received electromagnetic energy or theinformation representing, or derived from, at least part of the receivedelectromagnetic energy obtained by the at least one ground station, atleast one of: the composition of at least part of the receivedelectromagnetic energy; a location on earth of the origin of at leastpart of the received electromagnetic energy; a level of receivedelectromagnetic energy reaching at least part of the geostationaryorbit; and at least one characteristic of an uplink transmission. 29.The method of claim 28, wherein estimating the location comprisesestimating a longitude and latitude.
 30. The method of claim 28, whereinestimating a location on earth of the origin of at least part of thereceived electromagnetic energy comprises estimating the location of aninterference source.
 31. The method of claim 28, wherein estimating atleast one characteristic of an uplink transmission comprises estimatingan uplink pattern of at least one of an antenna on earth; and a group ofantennas on earth.
 32. A use of a satellite according to claim 1, orinformation obtained from a satellite according to claim 1, forestimating at least one of: the composition of at least part of thereceived electromagnetic energy; a location on earth of the origin of atleast part of the received electromagnetic energy; a level of receivedelectromagnetic energy reaching at least part of the geostationaryorbit; and at least one characteristic of an uplink transmission.